Atomic Force Microscopes (AFMs) belong to high resolution type of scanning probe microscopes with resolutions in the range of fractions of a nanometer. In the Atomic Force Microscopy, a microscale cantilever with a sharp tip (probe) at its end is used to scan the surface of a sample. When the tip is brought into proximity of a sample surface, the forces between the tip and the sample lead to a deflection of the cantilever in accordance with Hooke's Law. Typically, the deflection of the cantilever is measured to obtain the sample's topography.
The AFM may be operated in a number of imaging modes, including contact modes (also called static modes) and a variety of dynamic modes. In the contact mode of operation, shown in FIG. 1A, the static tip deflection is used as a feedback signal. Due to the tendency of a static signal to be noisy and drift, low stiffness cantilevers are used to boost the deflection signal. However, close to the surface of the sample, attractive forces may be strong, causing the tip to collide with the surface of the sample, thereby damaging the specimen and resulting in an inaccurate measurement.
In the dynamic (Non-Contact) mode, shown in FIG. 1B, the cantilever is externally oscillated at or close to its resonance frequency. The oscillation amplitude, phase and resonance frequency are modified by tip sample interaction forces. These changes in oscillation parameters with respect to the external reference oscillation provide information about the sample's characteristics. Schemes for dynamic mode operation include frequency modulation and the more common, amplitude modulation. In frequency modulation, changes in the oscillation frequency provide information about tip-sample interactions. In amplitude modulation, changes in the oscillation amplitude or phase provide a feedback signal for imaging.
Amplitude modulation may be operated either in the Non-Contact or in the Intermittent Contact regime, also known as a tapping mode, shown in FIG. 1C. In the tapping mode, the cantilever is periodically oscillated such that it comes in contact with the sample with each oscillation cycle, and then a restoring force is provided by the cantilever spring to detach the tip from the sample. In the tapping mode, the cantilever is driven to oscillate vertically near its resonance frequency by a small piezoelectric element mounted in the AFM tip holder (not shown in the Drawings). Due to the interaction forces acting on the cantilever when the tip comes close to the surface of the sample, the amplitude of the oscillation decreases.
An electronic servomechanism is typically used to control the height of the cantilever above the sample to maintain a continuous oscillation amplitude as the cantilever is scanned over the sample surface. The tapping AFM image (response) is produced by imaging the contact force between the tip and the sample surface.
For example, U.S. Pat. No. 5,412,980 describes a tapping Atomic Force Microscope in which a probe tip is oscillated at a resonance frequency and at an amplitude set point. The probe tip is scanned across the surface of a sample in contact with a sample, so that the amplitude of oscillation of the probe is changed in relation to the topography of the surface of the sample. The set point amplitude of oscillation of the probe is maintained greater than 10 nm to ensure that the energy in the cantilever is much higher than that lost in each cycle as the result of the impact of the probe of the sample surface, which avoids sticking of the probe tip to the sample surface. The imaging data either corresponds to a control signal produced to maintain the established amplitude or is a function of changes in the amplitude of oscillation of the probe.
The tapping mode of operation of the AFM as described in '980 Patent, is intended to avoid sticking of the probe tip to the sample surface. However, the problem may still persist when the tapping AFM is used for measurements of soft materials, i.e., biological specimen, where an elastic collision may occur, also known as “soft impact”. In this situation, the exact determination of the moment when collisions begin is not attainable. The failure to control the impact force in the precise manner may cause damage to the specimen or result in inaccurate measurements.
Therefore, it is desirable to precisely identify the moment of the beginning of the impact (aka a grazing event, when the probe just touches the sample surface with zero velocity) in order to operate the Atomic Force Microscope in the tapping mode with the reduced impact force between the probe and sample during a collision.